The present invention relates to making semiconductor components and more particularly relates to devices for growing epitaxial layers on substrates, such as wafers.
Various industries employ processes to form thin layers on solid substrates. The substrates having deposited thin layers are widely used in microprocessors, electro-optical devices, communication devices and others. The processes for the deposition of the thin layers on solid substrates are especially important for the semiconductor industry. In the manufacturing of semiconductors, the coated solid substrates, such as substantially planar wafers made of silicon and silicon carbide, are used to produce semiconductor devices. After the deposition, the coated wafers are subjected to well-known further processes to form semiconductor devices such as lasers, transistors, light emitting diodes, and a variety of other devices. For example, in the production of the light-emitting diodes, the layers deposited on the wafer form the active elements of the diodes.
The materials deposited on the solid substrates include silicon carbide, gallium arsenide, complex metal oxides (e.g., YBa2Cu3O7) and many others. The thin films of inorganic materials are typically deposited by the processes collectively known as chemical vapor deposition (CVD). It is known that the CVD processes, if properly controlled, produce thin films having organized crystal lattices. Especially important are the deposited thin films having the same crystal lattice structures as the underlying solid substrates. The layers by which such thin films grow are called the epitaxial layers.
In a typical chemical vapor deposition process, the substrate, usually a wafer, is exposed to gases inside a CVD reactor. Reactant chemicals carried by the gases are introduced over the wafer in controlled quantities and at controlled rates while the wafer is heated and usually rotated. The reactant chemicals, commonly referred to as precursors, are introduced into the CVD reactor by placing the reactant chemicals in a device known as a bubbler and then passing a carrier gas through the bubbler. The carrier gas picks up the molecules of the precursors to provide a reactant gas that is then fed into a reaction chamber of the CVD reactor. The precursors typically consist of inorganic components, which later form the epitaxial layers on the surface of the wafer (e.g., Si, Y, Nb, etc.), and organic components. Usually, the organic components are used to allow the volatilization of the precursors in the bubbler. While the inorganic components are stable to the high temperatures inside the CVD reactor, the organic components readily decompose upon heating to a sufficiently high temperature. When the reactant gas reaches the vicinity of a heated wafer, the organic components decompose, depositing the inorganic components on the surface of the wafer in the form of the epitaxial layers.
CVD reactors have various designs, including horizontal reactors in which wafers are mounted at an angle to the inflowing reactant gases; horizontal reactors with planetary rotation in which the reactant gases pass across the wafers; barrel reactors; and vertical reactors in which wafers are rotated at a relatively high speed within the reaction chamber as reactant gases are injected downwardly onto the wafers. The vertical reactors with high-speed rotation are among the most commercially important CVD reactors.
Among the desirable characteristics for any CVD reactor are heating uniformity, low reactor cycle time, good performance characteristics, longevity of the internal parts that are heated and/or rotated inside the reaction chamber, ease of temperature control and high temperature tolerance for component parts. Also important are the cost of the required component parts, ease of maintenance, energy efficiency and minimization of the heating assembly""s thermal inertia. For example, if the heated components of a CVD reactor have high thermal inertia, certain reactor operations may be delayed until the heated components reach the desired temperatures. Therefore, lower thermal inertia of the heated components of the reactor increases the productivity since the throughput depends upon the reactor cycle time. Similarly, if the internal parts of the reactor that are rotated during the deposition undergo even a small degree of deformation, the reactor may exhibit excessive vibration during use, resulting in heightened maintenance requirements.
A typical prior art vertical CVD reactor is illustrated in FIG. 1. As seen from FIG. 1, a wafer 10 is placed on a wafer carrier 12, which is placed on a susceptor 14. The wafer carrier 12 is usually made from a material that is relatively inexpensive and allows good manufacturing reproducibility. The wafer carrier may have to be replaced after a certain commercially suitable number of reactor cycles. The susceptor 14 is permanently mounted and supported by a rotatable spindle 16, which enables rotation of the susceptor 14, the wafer carrier 12 and the wafer 10. The susceptor 14, the wafer carrier 12 and the wafer 10 are located in an enclosed reactor chamber 18. A heating assembly 20, which may include one or more heating filaments 22, is arranged below the susceptor 14, and heated by passing an electric current through electrodes 25. The heating assembly 20 heats the susceptor 14, the wafer carrier 12 and, ultimately, the wafer 10. The rotation of the wafer carrier 12 is intended to enhance the temperature uniformity across the deposition area, as well as the uniformity of the reactant gas introduced over the wafer 10 during the deposition. As the wafer-supporting assembly (spindle/susceptor/wafer carrier) rotates the heated wafer 10, the reactant gas is introduced into the reaction chamber 18, depositing a film on the surface of the wafer 10.
The vertical CVD reactors having both the susceptor and the wafer carrier, similar to the reactor shown in FIG. 1, enjoy a widespread and successful use for a variety of CVD applications. For example, the Enterprise and Discovery reactors, made by Emcore Corporation of Somerset, N.J., are some of the most successful CVD reactors in the commercial marketplace. However, as discovered by the inventors of the present invention, the performance of such CVD reactors may be further improved for certain CVD applications.
First, the CVD reactor having both a susceptor and a wafer carrier contains at least two thermal interfaces. Referring to FIG. 1, these are the interfaces between the heating assembly 20 and the susceptor 14, and between the susceptor 14 and the wafer carrier 12. Research by the inventors of the present invention has shown that a substantial temperature gradient exists at these interfaces. For example, the temperature of the heating assembly 20 is higher than the temperature of the susceptor 14, which, in turn, is higher than the temperature of the wafer carrier 12. Consequently, the heating assembly 20 must be heated to a substantially higher temperature than the temperature desired for the wafer 10 during the deposition. The required higher temperatures of the heating assembly lead to higher energy consumption and faster deterioration of the heating assembly""s components. In addition, the typical susceptor possesses a significant heat capacity, and thus a large thermal inertia, substantially increasing the time required to heat and cool down the wafer carrier 12. This results in a longer reactor cycle and consequent reduction in the productivity of the reactor. Also, the inventors have determined that the longer reactor cycle time tends to result in a less precise and less flexible control of the wafer carrier""s temperature, increasing the time necessary to stabilize the temperature of the wafer carrier prior to the deposition.
Second, in the CVD reactors similar to the reactor of FIG. 1, the susceptor 14 must withstand a large number of reactor cycles since it is permanently mounted in the reaction chamber, and typically may not be easily replaced without interrupting the reactor cycle, opening up the reactor and removing the parts that permanently attach the susceptor to the spindle, such as screws, bolts and the like. Therefore, the susceptors are usually made from highly temperature- and deformation-resistant materials, typically molybdenum. Such materials are very expensive and often exhibit a high thermal inertia.
Third, every additional interface in the wafer-supporting assembly increases the manufacturing tolerance requirements. For example, again with reference to FIG. 1, the spacing between the susceptor 14 and the wafer carrier 12 must be precise and uniform to produce the required uniform heating of the wafer. However, notwithstanding the high precision machining used in the manufacturing of the susceptors, the susceptor/wafer carrier spacing is likely to exhibit some non-uniformity due to both the over-the-time deformation of the susceptor and a certain unavoidable degree of deviation in the susceptor-to-susceptor manufacturing reproducibility. Further, a small degree of deformation of the susceptor is essentially unavoidable in the CVD reactors having both the susceptor and the wafer carrier due to the required non-uniform heating of the susceptor to produce the uniform heating of the wafer carrier. The accumulated deformation of the susceptor eventually may lead to an excessive vibration of the wafer-supporting assembly during rotation in the deposition process, and the resulting loss and destruction of coated wafers.
Fourth, in the CVD reactors with permanently mounted susceptors, the susceptor is typically rigidly attached to the spindle to minimize the vibration during the operation of the reactor. The spindle/susceptor connection is heated during the repeated operation of the reactor and sometimes becomes difficult to disassemble, complicating the maintenance and the replacement procedures.
Finally, the heavier is the wafer-supporting assembly, the larger is the mechanical inertia of the spindle. In turn, the high mechanical inertia increases the strain on the spindle-supporting assembly, reducing its lifetime.
Notwithstanding these limitations, the existing prior art CVD reactors having both a susceptor and a wafer carrier continue enjoying a successful and widespread use in the semiconductor industry.
Nevertheless, there exists a need for a CVD reactor that minimizes these limitations of the presently available CVD reactors while maintaining a high level of performance.
The present invention addresses this need by providing a novel CVD reactor in which the wafer carrier is placed on the rotatable spindle without a susceptor, and a related method of growing epitaxial layers in a CVD reactor. These novel reactors are likely to be used along with the presently available successful CVD reactors, such as the reactor shown in FIG. 1.
It has been determined by the inventors that, in the prior art CVD reactors, for example, the prior art reactor shown in FIG. 1, substantial thermal losses occur at thermal interfaces in the wafer-supporting assembly. The research by the inventors also has shown that the increase in the temperature of the heating filament required to achieve the desired wafer temperature significantly reduces the lifetime of the heating filaments.
It has also been determined by the inventors that the presence of a permanently mounted susceptor in the prior art CVD reactors makes a significant contribution to the overall thermal and mechanical inertia of the wafer-supporting assembly.
The inventors have also determined that the rotatable spindle is a source of a substantial heat drain from the wafer-supporting assembly during the deposition. This heat drain may negatively affect the heating uniformity, the energy efficiency and the lifetime of the heating filaments.
Therefore, the present invention provides a novel CVD reactor, use of which minimizes these limitations of the presently available CVD reactors, as well as the limitations described in the Background section herein.
According to one aspect of the invention, an apparatus for growing epitaxial layers on one or more wafers by chemical wafer deposition is provided, and includes a reaction chamber, a rotatable spindle, a heating means for heating the wafers and a wafer carrier for supporting and transporting the wafers between a deposition position and a loading position.
In the loading position, the wafer carrier is separated from the rotatable spindle and the wafers may be placed on the wafer carrier for subsequent transfer to the deposition position. The loading position may be located inside the reaction chamber or outside the reaction chamber. Preferably, the loading position is located outside the reaction chamber. There may be one or more of such loading positions.
In the deposition position, the wafer carrier is detachably mounted on the rotatable spindle inside the reaction chamber, permitting chemical vapor deposition of the wafers placed on the wafer carrier. Preferably, in the deposition position, the wafer carrier is in direct contact with the spindle. Also, preferably, when in the deposition position, the wafer carrier is centrally mounted onto the spindle and supported only by the spindle. Most preferably, the wafer carrier is retained on the spindle by the force of friction, meaning that there exist no separate retaining means for retaining the wafer carrier on the spindle in the deposition position. However, the apparatus of the present invention may also include a separate retaining means for retaining the wafer carrier in the deposition position. The separate retaining means may be integral with the rotatable spindle or separate from both the spindle and the wafer carrier.
The wafer carrier of the invention may include a top surface and a bottom surface. The top surface of the wafer carrier may include one or more cavities for placing the wafers. The bottom surface may include a central recess for detachably mounting the wafer carrier onto the spindle. The central recess extends from the bottom surface of the wafer carrier toward the top surface of the wafer carrier to a recess end point. Preferably, the central recess does not reach the top surface of the wafer carrier and therefore the recess end point lies at a lower elevation than the top surface of the wafer carrier.
The rotatable spindle includes an upper end for mounting the wafer carrier inside the reaction chamber. In the deposition position, the upper end of the spindle is inserted into the central recess of the bottom surface of the wafer carrier. Preferably, to improve the rotational stability of the wafer carrier, the spindle supports the wafer carrier above the wafer carrier""s center of gravity.
The apparatus of the invention may also include a mechanical means for transporting the wafer carrier between the deposition position and the loading position. The heating means of the apparatus of the invention may include one or more radiant heating elements. The apparatus of the invention may be used to process a single wafer or a plurality of wafers.
According to another aspect of the present invention, an apparatus for growing epitaxial layers on one or more wafers by chemical vapor deposition is provided; the apparatus including a reaction chamber, a rotatable spindle having an upper end located inside the reaction chamber, a wafer carrier and a radiant heating element disposed under the wafer carrier. The wafer carrier provides a support and transports the wafers. During the deposition, the wafer carrier is centrally and detachably mounted on the upper end of the spindle, where it is in a contact with the spindle. The wafer carrier is mounted in a manner that allows it to be readily removed from the upper end of the spindle. After the deposition is complete or at any other time, the wafer carrier may be removed from the upper end of the spindle and transported to a position for loading or unloading wafers. There may be one or a plurality of such loading positions. The loading position may be located inside the reaction chamber or outside the reaction chamber. Preferably, the wafer carrier is in a direct contact with the upper end of the spindle and has a top surface that includes one or a plurality of cavities for supporting a plurality of wafers. Therefore, either a single wafer or a plurality of wafers may be deposited in the reactor of the invention at the same time. The wafer carrier is transported between the position mounted onto the upper end of the spindle and the loading position by mechanical means, typically a robotic arm.
In a preferred embodiment of this aspect of the invention, the bottom surface of the wafer carrier includes a central recess, which extends upward from the bottom surface in a direction of the top surface of the wafer carrier, terminating in a recess end point. The central recess does not reach the top surface of the wafer carrier. Therefore, the recess end point is located at a lower elevation than the top surface of the wafer carrier. When the wafer carrier is mounted onto the upper end of the spindle, the upper end of the spindle is inserted into the central recess in the bottom surface of the wafer carrier. The insertion provides a point of conduct between the spindle and the wafer carrier, allowing the wafer carrier to be supported by the spindle. To improve the rotational stability of the wafer carrier, the point of contact between the spindle and the wafer carrier having the highest elevation is located above the center of gravity of the wafer carrier.
In the most preferred embodiment of this aspect of the invention, the wafer carrier has a substantially round shape. In this embodiment, the top surface and the bottom surface of the wafer carrier are substantially parallel to each other. Of course, the top surface of the wafer carrier may include cavities for placing the wafers, and the bottom surface of the wafer carrier includes a recess for mounting the wafer carrier onto the upper end of the spindle, and other indentations or raised features are not excluded on either the top surface or the bottom surface of the wafer carrier.
The spindle according to this embodiment of the invention has a substantially cylindrical shape and an axis of rotation. The bottom surface of the wafer carrier, when mounted on the spindle, is substantially perpendicular to the axis of rotation of the spindle. The upper end of the spindle preferably terminates in a substantially flat top surface, which is also substantially perpendicular to the axis of rotation of the spindle. Preferably, the upper end of the spindle narrows toward the substantially flat top surface of the spindle. Therefore, the narrow portion of the upper end of the spindle is located near the substantially flat top surface of the spindle, and the wide portion of the spindle is located distal from the substantially flat top surface of the spindle.
As has been stated, the spindle is a source of a significant heat drain from the wafer-supporting assembly. The present invention provides the novel way of reducing this heat drain. To this end, in a preferred embodiment, the spindle has a cavity extending vertically downward from the substantially flat top surface of the upper end of the spindle to a cavity end point, which is disposed at a predetermined depth. The cavity in the spindle has a substantially cylindrical shape and is substantially coaxial with the spindle. The predetermined depth of the cavity in the spindle is preferably from about 3 to about 4 spindle diameters. This hollow construction of the upper end of the spindle allows the reduction of the heat drain from the wafer-supporting assembly.
To further reduce the heat drain, a specific arrangement of the radiant heating elements is provided. In this arrangement, the radiant heating element includes a first radiant heating element that is substantially coaxial with the rotatable spindle and has a top surface proximal to the bottom surface of the wafer carrier, an internal circumference and an external circumference. The internal circumference of the first radiant heating element defines a round opening around the spindle. This arrangement of the radiant heating elements of the invention may also include a second radiant heating element substantially coaxial with the first radiant heating element and the spindle, and located between the first radiant heating element and the spindle. The second radiant heating element defines an external circumference, the radius of which is smaller than the radius of the internal circumference of the first radiant heating element. Most preferably, the top surface of the second radiant heating element is located at substantially the same elevation as the top surface of the first radiant heating element, and the bottom surface of the second radiant heating element is located at the same elevation as the cavity end point of the rotatable spindle. The second radiant heating element allows heating of the upper end of the spindle, which along with the hollow construction of the upper end of the spindle reduces the heat drain from the wafer-supporting assembly. The reactor of the invention may also include a radiant heating shield.
According to yet another aspect of the invention, a method of growing epitaxial layers on one or more wafers by chemical wafer deposition is provided. According to the method of the invention, the chemical wafer deposition is carried out in a reactor chamber that includes a rotatable spindle having an upper end disposed inside the reaction chamber. To carry out the deposition, the method includes
a) providing a wafer carrier having a surface for retaining one or more wafers;
b) placing one or more wafers on the surface of the wafer carrier in a loading position, in which the wafer carrier is separated from the spindle;
c) transporting the wafer carrier towards the spindle;
d) detachably mounting the wafer carrier on the upper end of the spindle for rotation therewith; and
e) rotating the spindle and the wafer carrier located thereon while introducing one or more reactants to the reaction chamber and heating the wafer carrier.
Preferably, the method of the invention further includes removing the wafer carrier from the upper end of the spindle to unload the wafers. The step of detachably mounting the wafer carrier may include directly mounting the wafer carrier, and/or centrally mounting the wafer carrier on the upper end of the spindle. Preferably, the wafer carrier is mounted on the upper end of the spindle above the wafer carrier""s center of gravity and retained therein only by a force of friction. Preferably, the loading position is located outside the reaction chamber.